These metadata are specifically for the State of Maine, one state in the national compilation of state geologic map databases. The digital geologic state map data of Maine was prepared by the Maine Geological Survey in cooperation with the USGS between 1987-1994. Subsequent editing of the spatial data by the USGS was limited to fitting a standardized state boundary to the data, standardizing the arc coding, reprojecting the data, and minor edits of arc or polygon attributes to conform with the paper map.
This open-file report is one of several that present the preliminary results of the USGS Mineral Resources Program activity to compile a national-scale geologic map database to support national and regional level projects, including mineral resource and geoenvironmental assessments. The only comprehensive sources of regional- and national-scale geologic maps are state geologic maps with scales ranging from 1:100,000 to 1:1,000,000. Digital versions of these state maps form the core of what is presented here. Because no adequate geologic map exists for the state of Alaska, it is being compiled in regional blocks that also form part of this national database. It is expected that this series will completed by approximately the end of 2007. These maps and databases are being released in blocks of states or, in the case of Alaska, as compiled blocks of 1:250,000-scale quadrangles as chapters in this series. For Alaska, formal maps as well as databases are being published here, whereas for the conterminous U.S. only state databases and preview graphics are presented, because published maps for most states already exist. For Alaska these regional compilations will form the base for compiling a new geologic map of the state. As documented in CONUSdocumentationNENJ.pdf, standards for the conterminous U.S. are somewhat different than those for Alaska and Hawaii.

Planar coordinates are encoded using coordinate pair
Abscissae (x-coordinates) are specified to the nearest 0.01
Ordinates (y-coordinates) are specified to the nearest 0.01
Planar coordinates are specified in Meters
The horizontal datum used is North American Datum of 1927.
The ellipsoid used is Clarke 1866.
The semi-major axis of the ellipsoid used is 6378206.4.
The flattening of the ellipsoid used is 1/294.98.

A primary goal of this work is to develop geologic map datasets with standardized structure and attribution. The database design and standards are documented in CONUSdocumentationNENJ.pdf of this series. The intent is that contiguous databases can be merged seamlessly and without any additional effort. In addition to a common database structure, the conterminous U.S. state databases (the contiguous 48 states) have been fitted to a set of standard state boundaries so that, when states are merged, they match without slivers or overlap. No attempt has been made to reconcile differences in mapping across state boundaries.

The purpose of this standardization is to allow all SGMC covers to be seamlessly joined without any additional effort to form regional or national level digital maps. Note that for Alaska, the state is being compiled as blocks of 1:250k quadrangles that will ultimately be used to compile a new state map. Douglas Stoeser (USGS) and Ric Wilson (USGS) are co-coordinators for the overall SGMC effort, with Stoeser coordinating work on the mainland states and Wilson coordinating work for Alaska and Hawaii.

First step was to convert from a shapefile to a coverage. This was done in Arc/Info by using the shapearc command. Then the labels had to be attached to the polygons so the GET function was used in Arc/Info.

rocktype2 records second most dominant lithology, using standardized data dictionary

The attributes were then converted using the following structure:

item name: orig_label
width: 12
output: 12
type: c

item name: sgmc_label
width: 16
output: 16
type: c

item name: unit_link
width: 18
output: 18
type: c

item name: source
width: 6
output: 8
type: c

item name: unit_age
width: 60
output: 60
type: c

item name: rocktype1
width: 40
output: 40
type: c

item name: rocktype2
width: 40
output: 40
type: c

Date: 2006 (process 5 of 10)

Once the .dbf was in the correct format with standardized column widths and data structure, it was converted into a lookup table (.lut) in Arc/Info. Then a joinitem was performed to join the look-up table to the polygon attribute table. orig_label was used as the merge item.

Date: 2006 (process 6 of 10)

Next, in preparing the uniform .aat file, the following items were added to the table:

item name: arc-code
width: 3
output: 3
type: i

item name: arc-para1
width: 3
output: 3
type: i

item name: arc-para2
width: 3
output: 3
type: i

item name: source
width: 6
output: 8
type: c

Date: 2006 (process 7 of 10)

Arc-code was populated using the paper map source to tag the lines using the AAT line type data dictionary (see CONUSdocumentationNENJ.pdf in this series).

Arc-para1 was populated when additional information was available such as identifying the upthrown side of a fault or direction of thrusting on a fault. (see CONUSdocumentationNENJ.pdf)

Arc-para2 is a scratch field that is used mostly in Alaska.

Source was populated with an alphanumeric code (e.g. US001) that designates the source used for the linework.

Date: 2006 (process 8 of 10)

Once the .aat and the .pat were populated with the correct attributes, the coverage was exported as an .e00 file (MEgeol_lcc.e00).

Then it was unprojected into geographic coordinates and exported again (MEgeol_dd.e00).

Date: 2006 (process 9 of 10)

Faults were extracted from the coverage and created a separate fault coverage (MEfaults_lcc) and then exported (MEfaults_lcc.e00).

Date: 2006 (process 10 of 10)

Finally, the geology lines, geology polygons and faults were converted into shapefiles; in both geographic coordinates and in Lambert Conformal Conic projection.

How well have the observations been checked?
The values of the attributes ROCKTYPE1 and ROCKTYPE2 were assigned based on the information in the legends of the source maps (most of which are paper). The definitions of these values are described in the draft document Geologic Map Classification version 6.1 (Johnson and others); available at the following website: http://geology.usgs.gov/dm

How accurate are the geographic locations?
Accuracy of Maine state boundary and those arcs intersecting it: The accuracy of the state boundary arcs is based on the 1:100,000 scale original data used to produce this boundary. (Johnson, Bruce R. and Leveritch, Beth, 1998, 1:100,000 Scale State Boundaries of the Conterminous United States: U.S. Geological Survey, unpublished (see metadata file: st100kmeta.txt, CONUSdocumentationNENJ.pdf))

Those internal arcs intersecting the state boundary were extended or shortened as needed to complete polygons, possibly creating a small amount of error.

This database is not meant to be used or displayed at any scale other
than 1:500,000.

Any hardcopies utilizing these data sets shall clearly indicate
their source. If the licensee has modified the data in any way
they are obligated to describe the types of modifications they
have performed on the hardcopy map. Licensee specifically agrees
not to misrepresent these data sets, nor to imply that changes they
made were approved by the U.S. Geological Survey.

Any use of trade, product, or firm names is for
descriptive purposes only and does not imply endorsement by the USGS.
Although these data have been used by the USGS and have been successfully
imported into data base programs, no warranty, expressed or implied, is made
by the USGS as to how successfully or accurately the data can be imported into
any specific application software running on any specific hardware platform.
The fact of distribution shall not constitute any such warranty, and no
responsibility is assumed by the USGS in connection therewith.
This data base GIS is not meant to be used or displayed at any scale other than 1:500,000.